1. Field of the Invention
The present invention relates to an infrared video camera system for rendering video images of survey scenes containing gas leaks. More specifically, the invention is a method and apparatus for generating temporally filtered images of the survey scene, scaling the temporally filtered images to reduce signal values of the temporally filtered images and generating a difference image by subtracting the scaled temporally filter video images from current video images to improve the visibility of gas plumes in the difference image.
2. Description of the Related Art
Thermographic video cameras are used to survey scenes to detect gas leaks. Many industrial gas leaks are invisible to the human observer. However, industrial gases often absorb infrared radiation at known absorption bands. Hand held video thermography camera systems are commercially available to detect gas leaks by viewing infrared images of the gas leaks. The gas plumes can be distinguished from other regions of the scene image because the gas plumes absorb infrared radiation and therefore have a reduced radiosity compared with the other regions.
Early versions of gas leak detecting thermography cameras include an infrared illuminator to flood the survey scene with infrared radiation and to render video images of the scene using the backscattered illumination. In particular, the scene is illuminated at infrared wavelengths corresponding to an absorption band of a gas to be detected. Laser illuminators having a fixed infrared spectral bandwidth as well as laser illuminators having a variable spectral bandwidth have been used in gas detections systems. The camera system forms a video image of the survey scene using backscattered illumination. The video image shows absorbing gas plumes as dark areas having lower levels of backscattered infrared radiation than other areas of the scene. The infrared illumination increases the visibility of the gas plumes with respect to background elements. In addition, the camera systems may include a narrow spectral band pass filter to limit the spectral bandwidth of video images of the scene to a narrow spectral band approximately matching the absorption band of the gas to be detected. Examples of gas detecting thermography cameras that include an infrared illuminator for illuminating the survey scene at the spectral bandwidth of an absorption band of a gas being detected are disclosed in U.S. Pat. Nos. 4,555,672, 7,075,653, 6,995,846, 6,822,742, 7,151,787, 4,772,789, 7,134,322).
Backscattering thermography cameras have several drawbacks. These include the inability to view gas plumes against sky or other non-reflecting backgrounds, the need to use eye safe illuminators, and the limitation that the illuminator beam divergence limits the distance over which the illuminator can effective illuminate a scene. In addition, the cost and complexity of including an illuminator in the video camera system is an additional drawback.
More recently passive thermography cameras have been provided to image gas plumes without illuminating the survey scene. One example of a passive imaging device configured to detect the presence of methane and other hydrocarbon gas plumes is the video thermography camera disclosed in U.S. patent application Ser. No. 11/298,862, by Furry, which was published as US2006/0091310A1, and as WO2005001409, and which is incorporated by reference herein, in its entirety. A second example of a passive thermography camera configured to detect the presence of methane and other hydrocarbon gas plumes is commercially available from FLIR SYSTEM Inc. of Wilsonville, Oreg. and North Billerica, Mass., USA; sold under the trade name ThermaCam® GasFindIR™. A third example of a video thermography camera configured to detect the presence of the industrial gases having an absorption band approximately centered at 10.6 μm is disclosed in co-pending U.S. patent application Ser. No. 11/726,918, by Benson et al., filed on Mar. 23, 2007, which is incorporated by reference herein, in its entirety, and which is commonly assigned to the owner of the present invention. A fourth example of a passive thermography camera configured to detect the presence of sulfur hexafluoride (SF6), ammonia, (NH3), uranium hexafluoride (UF6) and other industrial gas plumes is commercially available from FLIR SYSTEM Inc. of Wilsonville, Oreg. and North Billerica, Mass., USA, and sold under the trade name GasFindIR LW™.
Since passive thermography cameras do not rely on illumination to flood a survey scene, they can detect gas plumes over greater distances, they avoid eye safety issues associated with illuminating survey scenes and they eliminate the cost and complexity of including an illuminator in the camera system. However, passive thermography cameras usually requires more strict control on the spectral bandwidth of the video image and the reduction of signal noise in order to generate video images with enough contrast between gas plumes and other areas of the survey scene video image.
To reduce the spectral bandwidth of a scene image formed by the camera system, a narrow spectral band filter is positioned between a camera lens system and its photo sensor. The spectral band pass filter narrows the spectral irradiance of a scene image formed by the lens onto the photo sensor to approximately match the scene image spectral bandwidth with an absorption bandwidth of a gas to be detected. While matching the spectral band width of the scene image to an absorption band of the gas to be detected improves the ability of the camera system to distinguish between background elements and absorbing gas plumes, the narrow bandwidth of the scene image significantly reduces its total irradiance resulting in the need to increase sensor gain to render a video image. However, the increased sensor gain also amplifies noise in the image signal.
To reduce non-scene thermal noise, passive thermography cameras include a cryocooler to lower the operating temperature of the photo sensor, the spectral band pass filter and other support structures that may emit infrared radiation over the absorption bandwidth of the gas to be detected. While cooling the camera elements reduces thermal noise enough to provide adequate performance in many applications, improvements in passive camera systems are still needed e.g. to identify a gas plume in a video image when the background of the video image is sky, water, snow, or other non-reflecting backgrounds. Accordingly there is still a need to increase the visibility of gas plumes in passive thermographic video images.
It is know to use temporal filtering to reduce random signal noise from scene images to enhance video images. Temporal filtering in video image processing uses a plurality of recent image frames, collected over a selected time period, e.g. 10-100 video frames, and averages, integrates or otherwise temporally filters signal values at each location of the scene image to generate a temporally filtered image having reduce signal noise. In some applications, displaying the temporally filtered image improves the visibility of certain elements in the scene. However, temporal filtering tends to smooth or blur dynamic or transient elements of a scene image and gas plumes are transient elements, especially at their outer edges.
It is also known to subtract consecutive image frames from each other to improve the visibility of dynamic or transient elements in a scene image. The resulting difference image tends to include only dynamic or transient elements that have changed from the prior image frame. However the difference image has zero signal values at locations where consecutive scene images are substantially unchanged and such images can be disorienting because the difference image only shows dynamic or transient elements and there is no way to relate the dynamic elements with unchanged elements from scene to scene. Moreover when there are no dynamic elements in the scene the resulting difference image signal is zero at all location and the video image is a blank screen.
One attempt at using a difference image in a gas detecting camera system is disclosed in U.S. Pat. No. 5,656,813, to Moore et al., entitled APPARATUS FOR IMAGING GAS. In the '618 patent two cameras are used to render separate video images of the same scene and the separate video images are combined and displayed on a single display device. The first video camera is configured to render a video image of the scene at infrared wavelengths. The second video camera is configured to render a black and white video image of the survey scene at visible wavelengths. The infrared camera includes image processing systems for generating a temporally filtering image to reduce random noise. The temporally filtered image is subtracted from current images of the survey scene and the resulting difference image emphasizes the dynamic or transient elements of the survey scene, which include gas plumes. However since the image subtraction removes non-varying background elements of the survey scene from the infrared video image, the visible camera image is combined with the video camera image to replace the background elements. The problem with the '618 camera system is that it is complex and requires a two camera systems.
Instead, it is desirable to provide a single camera system capable of combining temporal filtering to reduce random noise with image subtraction to enhance the visibility of dynamic elements without completely eliminating background elements from the video image.